1. Field of the Invention:
The present invention relates to thermosetting cationic latices, coating compositions prepared therefrom and a process for providing protective coatings for substrates, using said coating compositions.
2. Brief Description of the Prior Art:
Cationic latices are known in the art and their method of preparation involving free radical emulsion polymerization of monomers in water, in the presence of cationic surfactants, is also known. The free radical emulsion polymerization procedure generally results in high molecular weight polymeric products which are normally in the range of 100,000 to several million. In this molecular weight range, the latices possess good mechanical properties. It has, however, been found that the latices leave something to be desired as to film properties relating to protective durability, i.e., the ability of the coatings to maintain their integrity against chemicals, weathering and the like.
In areas such as paper and textile manufacture where protective durability is not an essential service condition, cationic latices in their thermoplastic form have found a ready application primarily due to their inherent adhesion to anionic substrates. In areas such as protective coatings manufacture, where protective durability, particularly weathering, is an essential service condition, the cationic latices in their thermoplastic form are less desirable. In order to prepare good coatings, particularly protective coatings, with properties of outdoor durability, it is desirable to use thermosetting compositions. Coating compositions comprising thermosetting latices are, moreover, advantaged in that their application avoids problems such as air pollution, fire hazards and other problem which attend solvent-based coating compositions. The art-known thermosetting latices of commercial significance are basically anionic, and not cationic.
In developing the present invention, it has been found that there are a number of attendant problems that might have, in the past, hindered the development of cationic thermosetting latices of commercial significance. In the synthesis of substantially coagulum-free and stable cationic latices, strong acids such as HCl, HBr and the like have been used, to provide surfactant gegen-ions. It is believed that certain acids adversely affect some of the properties of the resultant latices. It has, for example, been found that isocyante curing agents, when formulated with the art-known cationic latices, impart ineffective cure at ordinary cure temperatures.
In order to prepare effective thermosetting cationic latices, particularly those which will be useful as protective coatings especially in industrial applications, the above problems will have to be reduced or prefereably avoided. Unfortunately, the art-known thermosetting cationic latices are, by and large, still attended by the aforesaid problems. Seemingly, the prior art has failed to discover the source of the aforesaid problem and/or to provide solutions therefor.
By the present invention, there is now discovered the source of the problem relating to the inhibition of cure provided by isocyanate curing agents, and the solution therefor. Accordingly, there is now provided a thermosetting cationic latex, and coating compositions prepared therefrom which cure very effectively at ordinary baking temperatures.